Field of the Invention
The present invention relates to an optical system, and more specifically, to an endoscope applicable to medical and industrial usages.
Description of the Related Art
Optical systems (e.g., endoscopes) include medical endoscopes used for observing, diagnosing, and treating internal organs, and industrial endoscopes used for observation or repair of places in a mechanical apparatus or equipment, which are difficult to observe, such as interiors of pipes or gaps between instruments. These optical systems typically have a thickness from several mm to more than ten mm, and are configured to acquire images from a front to oblique directions of the endoscope by a wide angle optical system. In contrast, a configuration in which an image of a side surface of a void passage is acquired as disclosed in Japanese Patent Laid-Open No. 2008-309859 is known. The endoscopes of this type are capable of imaging an inner wall of the void passage located on a side surface of the endoscope.
However, when observing an inner wall in a thin tube, the endoscope by itself needs to be very thin. This requires a reduction in entire diameter including not only an observation optical system, but also an illuminating system that may be included in a lens barrel. However, the optical system disclosed in Japanese Patent Laid-Open No. 2008-309859 has a large diameter, and is not suitable for the endoscope as described above. Optical systems disclosed in Japanese Patent Laid-Open No. 2011-48086 and Japanese Patent Laid-Open No. 2011-17824 are allowed to reduce the diameter of the observation optical system by itself. However, a separate illuminating system is required, and hence there still remains a problem in reduction in diameter of the entire endoscope. Furthermore, since the separate illuminating system is required, the number of components is increased.
Due to the small sizes required, lens barrels of endoscopes cannot be easily processed with high form precisions like those of lens barrels of microscopes. In many cases, optical systems (endoscopes) are accommodated in cylindrical lens barrels and lenses are held by being positioned at a spacer. Japanese Patent Laid-Open No. 7-191267 discloses an exemplary endoscope in which lenses are positioned at a spacer and the lenses are bonded to and held by a cylindrical lens barrel.
However, when, as in Japanese Patent Laid-Open No. 7-191267, an optical system is held by a lens barrel, a space whose size equals the sum of the outside diameter of the optical system and the thickness of the lens barrel is required. Therefore, in applications in which diameters need to be reduced, the space for the lens barrel becomes an obstacle to reducing diameters.
In contrast, if an optical system is one in which lenses are held by being connected to a spacer without using a lens barrel, the space for the lens barrel is not required, so that such an optical system is suitable for reducing diameters. The lenses may be connected to the spacer by using a method that is often used for securing lenses to a lens barrel, that is, a method for bonding a peripheral portion of an optical surface to a spacer.
In performing the aforementioned bonding of the lenses, since, in an optical system having a small diameter, such as an endoscope, the lenses and the spacer are small, the amount of application of an adhesive is required to be very small. If the adhesive is accidentally applied by an amount that exceeds a prescribed amount, the adhesive protrudes outwardly from the lenses and may become an obstacle to reducing diameters. In addition, the protruded adhesive contacts members disposed at outer sides of the lenses and may cause undesired bonding to occur.
Thus, there is a need for new optical systems that can provide for observation of the side surfaces in a tubular passage as well as optical systems that can be manufactured having the small diameter required for these optical systems.